The present invention relates to a scanning optical system for a laser beam printer or the like, and particularly to a multi-beam scanning optical system using a plurality of beams.
Conventionally, a multi-beam scanning optical system has been widely employed. The multi-beam scanning optical system is advantageous in that a plurality of scanning lines can be formed simultaneously. The multi-beam scanning optical system typically includes a plurality of laser sources respectively emitting a plurality of laser beams. The plurality of laser beams are simultaneously deflected by a polygonal mirror. The deflected laser beams passes through an fxcex8 lens, which converges the plurality of laser beams on a surface, such as a photoconductive surface of a photoconductive drum, to be scanned to form a plurality of beam spots. As the polygonal mirror rotates, the beam spots formed on the photoconductive drum move to form a plurality of scanning lines thereon. The direction in which the beam spots move is parallel with the rotational axis of the photoconductive drum. Further, the photoconductive drum is rotated so that the photoconductive surface thereof is two-dimensionally exposed to the plurality of beams.
In this specification, a direction in which the beam spots move (i.e., a direction in which the scanning lines extend) will be referred to as a main scanning direction. Further, a direction in which the surface to be scanned moves with respect to the scanning lines, i.e., the rotation direction of the photoconductive drum will be referred to as an auxiliary scanning direction. In the following description, the shape of optical elements, directions of powers of the optical elements and the like are described with reference to the main and auxiliary scanning directions on the surface to be scanned. That is, if an optical element is described to have a refractive power in the main scanning direction, the power affects the beam in the main scanning direction on the surface to be scanned regardless of the orientation of the element.
In the multi-beam scanning optical system, all the beam spots should move within (i.e., traverse) a width of an imaging area so that the imaging area can be exposed to the beams. If the plurality of beam spots are arranged to align obliquely with respect to the main scanning direction, the scanning lines formed by the plurality of beam spots are shifted with each other in the main scanning direction. In such a case, it becomes necessary to elongate a width of each scanning line so that each beam traverses the imaging area. In order to elongate the scanning lines, it becomes necessary to use a larger polygonal mirror to broaden a deflection angle at which each beam scans. In view of a recent trend of downsizing of the imaging apparatus, it is not preferable to have such a configuration, and the plurality of beams are preferably aligned along a line which is perpendicular to the main scanning direction.
Generally, a scanning optical system is provided with a synchronizing signal detecting optical system for detecting a scanning position of each beam, which is used for controlling an imaging start point of each scanning line.
A typical synchronizing signal detecting optical system includes a photo sensor which detects a laser beam before it enters the imaging area. A predetermined period after the photo sensor detects the laser beam, modulation of the laser beam is started so that the image is formed from the imaging start position (i.e., the upstream end of the imaging area). If all of the plurality of beams are located at the same position in the main scanning direction, all the laser beams are incident on the photo sensor at the same time. Then, a single pulse signal is output by the photo sensor as the synchronizing signal. In such a case, all of the plurality of beams are started to be modulated after the same predetermined period has passed after the output of the pulse signal.
Practically, it is difficult to arrange the plurality of scanning lines at the same positions in the main scanning direction. It is because, all the beam spots are difficult to be aligned at an initial setting stage, and/or due to an external reason such as an oscillation at use, the relative positions of the plurality of beams may be changed to shift from each other in the main scanning direction. If two beam spots are slightly shifted in the main scanning direction, two pulse signals are output by the photo sensor within a very short period of time. In such a case, whichever pulse signal is used as the synchronizing signal, one of the two beam spots is not started to be modulated accurately, and therefore, the imaging start point of one of the two scanning lines is shifted from the predetermined position.
It is therefore an object of the invention to provide an improved multi-beam scanning optical system in which a plurality of scanning lines formed by a plurality of beams are aligned in the main scanning direction, and further, an imaging start point of each scanning line can be adjusted accurately.
For the above object, according to the present invention, there is provided a scanning optical system used for exposing a predetermined imaging area on a surface to be scanned to a plurality of laser beams. The scanning optical system is provided with a plurality of light sources that emit a plurality of laser beams having different wavelengths, respectively, a single deflector which deflects the plurality of laser beams simultaneously, an imaging optical system that converges the plurality of laser beams deflected by the single deflector on the surface to be scanned, lateral chromatic aberration of the imaging optical system being compensated, and a beam detector that receives the plurality of laser beams directed to outside the predetermined imaging area, a synchronizing signal being generated upon detection of each of the plurality of light beams by the beam detector. Further, the scanning optical system includes a dispersion element inserted in optical paths of the laser beams directed to the beam detector, the dispersion element being configured such that the laser beams directed to the beam detector are shifted in the scanning direction.
According to the optical scanning systems configured as above, beam spots formed on the surface to be scanned is aligned in the scanning direction within the imaging area, while the timings at which the beams traverse the light receiving element are differentiated so that synchronizing signals for respective beams can be generated. Therefore, the imaging start position can be accurately adjusted. Further, the imaging start points for the two laser beams can be adjusted in the main scanning direction even if the relative positions of the beam spots are shifted due to the error in the initial settings or some external disturbance.
Optionally, the beam detector detects the laser beams passed through at least a part of the imaging optical system. That is, the imaging optical system may include a plurality of lens elements, and the beams directed to the light receiving element may not pass through all of the lens elements.
In a particular case, the beam detector may include a single light receiving element, each of the plurality of laser beams being incident on the single light receiving element.
In this case, the plurality of laser beams incident on the imaging area are aligned in the scanning direction, and wherein the plurality of laser beams are incident on the single light receiving element at different timings.
Further optionally, the dispersion element may include a prism. Alternatively, the dispersion element may include an optical element formed with a diffraction surface which separates the plurality of laser beams in the scanning direction.
In the above case, the dispersion element may include a reflection type optical element that bends optical paths of the plurality of laser beams. By appropriately bending the optical paths of the beams which are used for generating the synchronizing signals, a space required for accommodating the entire scanning optical system can be reduced.